test: add focused coverage for DateValue and MQTT model paths

[augard]

Summary

• replace placeholder KiaTests.swift examples with focused unit/regression tests for recent PR ✨ Integrate MQTT real-time updates and improve authentication #6 model changes
• add coverage for DateValue decode/encode paths in DatePropertyWrapper.swift
• add coverage for MQTT protocol topic mapping and request encoding in MQTTModels.swift

Tests added

• timestamp milliseconds string decoding
• native Date decoding path via decoder strategy
• invalid timestamp parse failure path
• millisecond date formatter encode/decode round-trip
• MQTT subscriptionName mapping
• MQTT init(topicName:) success and unknown topic rejection
• ProtocolSubscriptionRequest raw-value JSON encoding
• MQTTConnectionStatus.displayText mapping

Verification

• tests introduced in KiaTests/KiaTests.swift
• local simulator-based xcodebuild test was not run in the previous sandboxed environment

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